Optical storage media are media in which data are stored in an optically readable manner, so that they can be read for example by means of a laser and a photodetector being integrated in a pickup. The detector is used for detecting reflected light of the laser beam when reading data from the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store up to about 50 GB on a dual-layer disc. For reading and writing of a Blu-ray Disc an optical pickup with a laser wavelength of 405 nm and a numerical aperture of 0,85 is used. On the Blu-ray Disc a track pitch of 320 nm and a mark length from 2T to 8T or 9T is used, where T is the channel bit length and wherein 2T corresponds with a minimum mark length of 138-160 nm.
The diffraction based resolution limit of static optical instruments as described by the Abbe theory is about lambda/2NA, which would be 238 nm for a Blu-ray type pickup having a laser wavelength lambda=405 nm and a numerical aperture NA=0,85. For the read-out of the high frequency (HF) data signal of a rotating Blu-ray Disc, a resolution of twice that value can be obtained. This minimal detectable length according to the diffraction theory corresponds to a period of the pattern function, which is formed of a pit and of a land having the same length. Thus, the smallest detectable element of such a system is a pit or a land having a length of about lambda/4NA, which is 119 nm for a Blu-ray type pickup.
New optical storage media with a super-resolution material offer the possibility of increasing the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-ray Disc. This is possible by including a nonlinear layer, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of the light spot used for reading from or writing to the optical storage medium. The nonlinear layer can be understood as a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam; this center reflectivity depends on the pit structure of the corresponding data layer. Therefore, the super-resolution effect allows to record and read data stored in marks or pits of an optical disc that have a size below the optical resolution limit of lambda/4NA of a corresponding optical pickup.
The nonlinear layer is often called a super-resolution near-field structure (SuperRENS) layer or super-resolution layer because it has been assumed that for some specific materials, the optical effect of reducing the effective spot size of the laser beam is based on a near-field interaction between the marks and spaces of the data layer and the nonlinear layer. SuperRENS optical discs comprising a super resolution near-field structure formed of a metal oxide, a polymer compound or a phase change layer comprising e.g. a GeSbTe or a AgInSbTe are known.